Advertisers are willing to pay more for ads on the iPad than on the iPhone, but both top Android

Apple's iOS continued to deliver more revenue per advertising impression to content publishers than any other mobile operating system in the quarter that ended in June, according to an Opera Software report.

IOS edged out Android in the important metric of eCPM, or the total revenue a publisher earns per thousand ads served in his or her content. Generally, this metric reflects a combination of what advertisers pay a publisher to display an ad and what they pay when users click on the ad. Higher eCPMs are good news for platform providers because they attract publishers to the platform.

IOS delivered an eCPM of $2.49, compared with $2.10 per thousand impressions on Android devices, according to the Opera report. Other platforms fell far behind the two leaders.

"Google has some catching up to do, but I wouldn't necessarily count them out," said Mahi de Silva, executive vice president of consumer mobile at Opera.

The report analyzed data from Opera's own mobile ad platform, which the company said serves 35 billion ad impressions per month for more than 9,000 customers around the world and accounts for $240 million in revenue for mobile publishers. Although Opera is perhaps best known for its mobile browser, its mobile advertising platform delivers ads primarily in apps, according to de Silva.

De Silva credited the difference in eCPM between the two platforms to smoother technical performance of ads in iOS, thanks to Apple's tight integration between hardware and software.

"There's some performance lag on Android. But clearly Google will close that gap," de Silva said.

Ads served on the iPad were most lucrative of all, earning publishers an eCPM of $3.96, more than $1 above earnings per thousand ads on the iPhone, which also runs iOS. But advertisements on the iPhone delivered $2.85 per thousand, which is still significantly more than those on Android.

The study said ads on devices with larger screens and touchscreens tend to earn publishers more revenue. Those that support user interaction with ads, such as clicking a phone number in the ad to initiate a call, also tend to be more lucrative.

As a result, tablets will become even more important as an advertising platform in the next six months, the study said, noting that demand for advertising on the devices has risen 140 percent since 2011.

Finance-related content publishers' earnings dramatically outpaced those of social content publishers, the study found. Users of financial apps are generally more affluent, de Silva said.

While ads in financial apps and websites accounted for just 6 percent of impressions, they accounted for 31 percent of revenue. Nearly a quarter of the ads served through Opera's mobile advertising network were in social apps and websites, but they brought in just 5 percent of revenue for content publishers.

The researchers predicted that the disparity between types of content will continue, but they "remain optimistic about social networking as a revenue driver over the long term," the report said.

ECPM for Research In Motion's operating system was $0.64. It was just $0.20 for Windows Phone 7, which the study attributes to low adoption rates.